It is known that a polymerization of ethylene can be effected by using a highly active catalyst composed of a solid catalytic ingredient in which a titanium tetrahalide is carried on a magnesium compound and another ingredient which consists of an organic aluminum compound. With respect to the above-mentioned polymerization, various approaches have been attempted to provide a new method which is effective for increasing the yield of the resultant polymer per unit weight of the solid catalytic ingredient in the catalyst used, to such an extent that the resultant polymer contains a very small amount of the used catalyst therein. As a result, the step of removing the used catalyst from the resultant polymer can be omitted.
For example, Japanese Patent Application Laid-Open No. 53-78287 (1978) discloses that ethylene can be polymerized by using a catalyst composed of a solid catalytic component which has been prepared by reacting a magnesium alcoholate with a reaction product of an aluminum halide with a tetraalkoxysilane, and by bringing the resultant solid product into contact with titanium tetrahalide, and another catalytic component consisting of an organic aluminum compound. Also, the above-mentioned laid-open application discloses that the pressure of ethylene in the polymerization procedure is adjusted to 10 kg/cm.sup.2, resulting in a yield of polyethylene of about 25,000 g per gram of the solid catalytic component per hour of polymerization time. However, in order to produce a polyethylene having a high degree of purity thereof so that the operation of removing the used catalyst from the resultant polymer is unnecessary, it is necessary to increase the yield of the polymer per gram of the used catalyst to more than the above-mentioned yield.
In another approach which was made by the inventors of the present invention, a method for polymerizing ethylene in the presence of an improved catalyst was attempted. The catalyst was prepared by reacting a Grignard compound with a reaction product of a tetraalkoxysilane with a aluminum halide, by bringing the resultant solid reaction product into contact with a titanium tetrahalide, and by mixing the resultant solid catalytic ingredient with another catalytic ingredient consisting of a trialkylaluminum. When ethylene was polymerized in the presence of the above-mentioned catalyst under a pressure of ethylene of 10 kg/cm.sup.2 G, the yield of polyethylene was from about 43,000 g to about 62,000 g per gram of the solid catalytic ingredient per hour of polymerization time.
It is known that in the polymerization of ethylene in the presence of a catalyst in which a magnesium compound is carried, the yield of the polyethylene per gram of the solid catalytic ingredient in the catalyst and per hour of polymerization time is proportional to the pressure of ethylene in the polymerization system. That is, the larger the pressure of ethylene, the larger the yield of the resultant polyethylene. However, high pressure polymerization is dangerous and, therefore, has to be carried out in a special pressure-resistant polymerization apparatus.
Accordingly, it is desirable to provide a new type of catalyst which is effective for polymerizing ethylene under a relatively low pressure of ethylene and for obtaining a satisfactory yield of the resultant polyethylene.